1. Field of the Invention
The present invention relates to a printing apparatus and control method therefor, and particularly to a printing apparatus which prints using an inkjet printhead, and a control method therefor.
2. Description of the Related Art
An inkjet printing apparatus generally comprises a carriage which supports a printhead and ink tank, a conveyance means for conveying a printing medium such as printing paper, and a control means for controlling the carriage and conveyance means. The inkjet printing apparatus prints by discharging ink onto a printing medium while the printhead having a plurality of printing elements (to be also referred to as nozzles hereinafter) for discharging ink droplets moves in a direction (main scanning direction) perpendicular to the printing medium conveyance direction (sub-scanning direction). At this time, the printhead in which many nozzles for discharging ink are arranged in line in the sub-scanning direction scans the printing medium once, thereby printing by a width corresponding to the number of nozzles.
In a conventional inkjet printing apparatus, in order to prevent ink from sticking to the internal wall of the nozzle of a printhead, the ink discharge surface of the printhead is generally capped to prevent evaporation of the ink solvent when performing no printing operation. Even during the printing operation, however, some of nozzles are hardly used or hardly discharge ink. These nozzles are exposed to the atmosphere for a long time without discharging ink while they are not capped with the ink-sticking prevention cap. As a result, the ink solvent evaporates from these nozzles, and ink sticks to their internal walls.
The stuck ink clogs the nozzle, causing an ink discharge failure. Conventionally, clogging prevention control (to be referred to as preliminary discharge hereinafter) is executed to drive a printing element once at a predetermined time interval, for example several seconds, discharge old ink from the nozzle, and supply new ink to it even during the printing operation. However, when ink is discharged from all the nozzles of the printhead at a predetermined time interval, the running cost rises and the throughput decreases.
To prevent this, there is disclosed an inkjet printing apparatus having a plurality of counting means for counting the numbers of ink discharges from all nozzles, and a determination means for determining whether all nozzles have performed a predetermined number of ink discharges at a predetermined time interval T (Japanese Patent Laid-Open No. 2004-58528). If no predetermined number of ink discharges have been performed at the predetermined time interval T, T is subtracted from a time T′ during which the printing operation can continue without performing preliminary discharge. If no predetermined number of ink discharges have still been performed, the difference between T′ and the time during which no predetermined number of ink discharges have been performed becomes “0”. Only when this difference becomes “0”, preliminary discharge is performed. Since preliminary discharge is performed only when necessary, an increase in running cost and a decrease in throughput can be suppressed.
Recently, inkjet printing apparatuses have been required to print at higher speeds with higher image qualities. To meet this requirement, inkjet printing apparatuses perform high-speed, high-quality printing by increasing the number of nozzles of the printhead.
To implement high-quality, high-resolution printing, various proposals have been made for even the nozzle arrangement of the printhead.
FIG. 6 is a view showing a specific example of the nozzle arrangement of a printhead.
In the example shown in FIG. 6, for each of black (Bk), cyan (C), magenta (M), and yellow (Y) inks, the printhead has two nozzle arrays for discharging ink of each color. The nozzle arrays for discharging ink of each color are arranged symmetrically with respect to the array direction of the nozzle arrays. The printhead using the nozzle arrays of this arrangement reciprocates to print by discharging ink in the forward and backward scanning directions not to create the difference between colors printed by forward scanning and backward scanning.
FIG. 7 is a view showing another specific example of the nozzle arrangement of a printhead.
Similar to the printhead in FIG. 6, the printhead shown in FIG. 7 has two nozzle arrays for discharging ink of each color (FIG. 7 shows only an example of discharging cyan ink). In the printhead shown in FIG. 7, two nozzle arrays of nozzles arranged at 600 dpi are arranged at an interval of 1,200 dpi with a shift of a half pitch in the nozzle array direction (longitudinal direction in FIG. 7). The printhead having the nozzle arrays of this arrangement can print at high resolution, particularly in the nozzle array direction.
However, according to these conventional techniques, as the number of nozzles increases, the number of counters for counting the number of ink discharges also increases. The larger number of nozzles leads to a larger circuit scale. FIG. 3 is a circuit diagram showing an example of a circuit having counters in a printing apparatus described in Japanese Patent Laid-Open No. 2004-58528. The circuit shown in FIG. 3 is arranged for each nozzle array of the printhead. As is apparent from this circuit arrangement, as the number of nozzles increases, the number of counters (Counters) increases.